P2Y₆ receptor inhibition perturbs CCL2-evoked signalling in human monocytic and peripheral blood mononuclear cells

Systematic P2Y receptor survey identifies P2Y11 as modulator of immune responses and virus replication in macrophages

The immune system is in place to assist in ensuring tissue homeostasis, which can be easily perturbed by invading pathogens or nonpathogenic stressors causing tissue damage. Extracellular nucleotides are well known to contribute to innate immune signaling specificity and strength, but how their signaling is relayed downstream of cell surface receptors and how this translates into antiviral immunity is only partially understood. Here, we systematically investigated the responses of human macrophages to extracellular nucleotides, focusing on the nucleotide-sensing GPRC receptors of the P2Y family. Time-resolved transcriptomic analysis showed that adenine- and uridine-based nucleotides induce a specific, immediate, and transient cytokine response through the MAPK signaling pathway that regulates transcriptional activation by AP-1. Using receptor trans-complementation, we identified a subset of P2Ys (P2Y1, P2Y2, P2Y6, and P2Y11) that govern inflammatory responses via cytokine induction, while others (P2Y4, P2Y11, P2Y12, P2Y13, and P2Y14) directly induce antiviral responses. Notably, P2Y11 combined both activities, and depletion or inhibition of this receptor in macrophages impaired both inflammatory and antiviral responses. Collectively, these results highlight the underappreciated functions of P2Y receptors in innate immune processes.

Pannexin1: insight into inflammatory conditions and its potential involvement in multiple organ dysfunction syndrome

Sepsis represents a global health concern, and patients with severe sepsis are at risk of experiencing MODS (multiple organ dysfunction syndrome), which is associated with elevated mortality rates and a poorer prognosis. The development of sepsis involves hyperactive inflammation, immune disorder, and disrupted microcirculation. It is crucial to identify targets within these processes to develop therapeutic interventions. One such potential target is Panx1 (pannexin-1), a widely expressed transmembrane protein that facilitates the passage of molecules smaller than 1 KDa, such as ATP. Accumulating evidence has implicated the involvement of Panx1 in sepsis-associated MODS. It attracts immune cells via the purinergic signaling pathway, mediates immune responses via the Panx1-IL-33 axis, promotes immune cell apoptosis, regulates blood flow by modulating VSMCs' and vascular endothelial cells' tension, and disrupts microcirculation by elevating endothelial permeability and promoting microthrombosis. At the level of organs, Panx1 contributes to inflammatory injury in multiple organs. Panx1 primarily exacerbates injury and hinders recovery, making it a potential target for sepsis-induced MODS. While no drugs have been developed explicitly against Panx1, some compounds that inhibit Panx1 hemichannels have been used extensively in experiments. However, given that Panx1's role may vary during different phases of sepsis, more investigations are required before interventions against Panx1 can be applied in clinical. Overall, Panx1 may be a promising target for sepsis-induced MODS. Nevertheless, further research is needed to understand its complex role in different stages of sepsis fully and to develop suitable pharmaceutical interventions for clinical use.

Mechanisms of sympathoexcitation via P2Y6 receptors

Many drugs used in cardiovascular therapy, such as angiotensin receptor antagonists and beta-blockers, may exert at least some of their actions through effects on the sympathetic nervous system, and this also holds true for e.g., P2Y12 antagonists. A new target at the horizon of cardiovascular drugs is the P2Y6 receptor which contributes to the development of arteriosclerosis and hypertension. To learn whether P2Y6 receptors in the sympathetic nervous system might contribute to actions of respective receptor ligands, responses of sympathetic neurons to P2Y6 receptor activation were analyzed in primary cell culture. UDP in a concentration dependent manner caused membrane depolarization and enhanced numbers of action potentials fired in response to current injections. The excitatory action was antagonized by the P2Y6 receptor antagonist MRS2578, but not by the P2Y2 antagonist AR-C118925XX. UDP raised intracellular Ca2+ in the same range of concentrations as it enhanced excitability and elicited inward currents under conditions that favor Cl- conductances, and these were reduced by a blocker of Ca2+-activated Cl- channels, CaCCInh-A01. In addition, UDP inhibited currents through KV7 channels. The increase in numbers of action potentials caused by UDP was not altered by the KV7 channel blocker linopirdine, but was enhanced in low extracellular Cl- and was reduced by CaCCInh-A01 and by an inhibitor of phospholipase C. Moreover, UDP enhanced release of previously incorporated [3H] noradrenaline, and this was augmented in low extracellular Cl- and by linopirdine, but attenuated by CaCCInh-A01. Together, these results reveal sympathoexcitatory actions of P2Y6 receptor activation involving Ca2+-activated Cl- channels.

Pannexin Channel Regulation of Cell Migration: Focus on Immune Cells

The role of Pannexin (PANX) channels during collective and single cell migration is increasingly recognized. Amongst many functions that are relevant to cell migration, here we focus on the role of PANX-mediated adenine nucleotide release and associated autocrine and paracrine signaling. We also summarize the contribution of PANXs with the cytoskeleton, which is also key regulator of cell migration. PANXs, as mechanosensitive ATP releasing channels, provide a unique link between cell migration and purinergic communication. The functional association with several purinergic receptors, together with a plethora of signals that modulate their opening, allows PANX channels to integrate physical and chemical cues during inflammation. Ubiquitously expressed in almost all immune cells, PANX1 opening has been reported in different immunological contexts. Immune activation is the epitome coordination between cell communication and migration, as leukocytes (i.e., T cells, dendritic cells) exchange information while migrating towards the injury site. In the current review, we summarized the contribution of PANX channels during immune cell migration and recruitment; although we also compile the available evidence for non-immune cells (including fibroblasts, keratinocytes, astrocytes, and cancer cells). Finally, we discuss the current evidence of PANX1 and PANX3 channels as a both positive and/or negative regulator in different inflammatory conditions, proposing a general mechanism of these channels contribution during cell migration.

On the mechanism of anti-CD39 immune checkpoint therapy

With the coming of age of cancer immunotherapy, the search for new therapeutic targets has led to the identification of immunosuppressive adenosine as an important regulator of antitumor immunity. This resulted in the development of selective inhibitors targeting various components of the adenosinergic pathway, including small molecules antagonists targeting the high affinity A2A adenosine receptor and low affinity A2B receptor, therapeutic monoclonal antibodies (mAbs) and small molecules targeting CD73 and therapeutic mAbs targeting CD39. As each regulator of the adenosinergic pathway present non-overlapping biologic functions, a better understanding of the mechanisms of action of each targeted approach should accelerate clinical translation and improve rational design of combination treatments. In this review, we discuss the potential mechanisms-of-action of anti-CD39 cancer therapy and potential toxicities that may emerge from sustained CD39 inhibition. Caution should be taken, however, in extrapolating data from gene-targeted mice to patients treated with blocking anti-CD39 agents. As phase I clinical trials are now underway, further insights into the mechanism of action and potential adverse events associated with anti-CD39 therapy are anticipated in coming years.

Taspine is a natural product that suppresses P2X4 receptor activity via phosphoinositide 3-kinase inhibition

BACKGROUND & PURPOSE

P2X4 is a ligand-gated cation channel activated by extracellular ATP, involved in neuropathic pain, inflammation and arterial tone.

EXPERIMENTAL APPROACH

Natural products were screened against human or mouse P2X4 activity using fura-2 loaded 1321N1 cells for measurement of intracellular Ca²⁺ responses; whole-cell currents were measured by patch clamp electrophysiological. Human primary macrophage chemokine release was used to assess effect of taspine on inflammatory cell function. An enzymatic assay was performed to assess the effect of taspine on recombinant PI3-kinase.

KEY RESULTS

A natural product screen identified taspine as an inhibitor of human P2X4 activity. Taspine inhibits human and mouse P2X4-mediated Ca²⁺ influx in 1321N1 cells expressing receptors (IC₅₀ 1.6±0.4 μM and 1.6±0.3 μM, respectively), but lacked activity at human P2X2, P2X3, P2X2/3 and P2X7 receptors. Taspine inhibited the maximal response at human and mouse P2X4 but had no effect on ATP potency. Taspine has a slow onset rate (~15 mins for half-maximal inhibition), irreversible over 30 minutes of washout. Taspine inhibits P2X4-mediated Ca²⁺ signalling in mouse BV-2 microglia cells and human primary macrophage. Taspine inhibited P2X4-mediated CXCL5 secretion in human primary macrophage. Taspine reversed ivermectin-induced potentiation of P2X4 currents in 1321N1 stably expressing cells. The known PI3-kinase inhibitor LY294002 mimicked the properties of taspine on P2X4-mediated Ca²⁺ influx and whole-cell currents. Taspine directly inhibited the enzymatic activity of recombinant PI3-kinase in a competitive manner.

CONCLUSIONS AND IMPLICATIONS

Taspine is a novel natural product P2X4 inhibitor, mediating its effect through PI3-kinase inhibitor rather than receptor antagonism. Taspine can inhibit the pro-inflammatory signalling by P2X4 in human primary macrophage.

Control of Macrophage Inflammation by P2Y Purinergic Receptors

Macrophages comprise a phenotypically and functionally diverse group of hematopoietic cells. Versatile macrophage subsets engage to ensure maintenance of tissue integrity. To perform tissue stress surveillance, macrophages express many different stress-sensing receptors, including purinergic P2X and P2Y receptors that respond to extracellular nucleotides and their sugar derivatives. Activation of G protein-coupled P2Y receptors can be both pro- and anti-inflammatory. Current examples include the observation that P2Y14 receptor promotes STAT1-mediated inflammation in pro-inflammatory M1 macrophages as well as the demonstration that P2Y11 receptor suppresses the secretion of tumor necrosis factor (TNF)-α and concomitantly promotes the release of soluble TNF receptors from anti-inflammatory M2 macrophages. Here, we review macrophage regulation by P2Y purinergic receptors, both in physiological and disease-associated inflammation. Therapeutic targeting of anti-inflammatory P2Y receptor signaling is desirable to attenuate excessive inflammation in infectious diseases such as COVID-19. Conversely, anti-inflammatory P2Y receptor signaling must be suppressed during cancer therapy to preserve its efficacy.

Purinergic Signaling in Controlling Macrophage and T Cell Functions During Atherosclerosis Development

Atherosclerosis is a hardening and narrowing of arteries causing a reduction of blood flow. It is a leading cause of death in industrialized countries as it causes heart attacks, strokes, and peripheral vascular disease. Pathogenesis of the atherosclerotic lesion (atheroma) relies on the accumulation of cholesterol-containing low-density lipoproteins (LDL) and on changes of artery endothelium that becomes adhesive for monocytes and lymphocytes. Immunomediated inflammatory response stimulated by lipoprotein oxidation, cytokine secretion and release of pro-inflammatory mediators, worsens the pathological context by amplifying tissue damage to the arterial lining and increasing flow-limiting stenosis. Formation of thrombi upon rupture of the endothelium and the fibrous cup may also occur, triggering thrombosis often threatening the patient’s life. Purinergic signaling, i.e., cell responses induced by stimulation of P2 and P1 membrane receptors for the extracellular nucleotides (ATP, ADP, UTP, and UDP) and nucleosides (adenosine), has been implicated in modulating the immunological response in atherosclerotic cardiovascular disease. In this review we will describe advancements in the understanding of purinergic modulation of the two main immune cells involved in atherogenesis, i.e., monocytes/macrophages and T lymphocytes, highlighting modulation of pro- and anti-atherosclerotic mediated responses of purinergic signaling in these cells and providing new insights to point out their potential clinical significance.

Phosphoinositide 3-kinase γ deficiency attenuates kidney injury and fibrosis in angiotensin II-induced hypertension

BACKGROUND

We have shown that the CXCL16/CXCR6 axis plays a critical role in recruiting inflammatory cells and bone marrow-derived fibroblasts into the kidney leading to renal injury and fibrosis. However, the underlying signaling mechanisms are not known.

METHODS

In the present study, we examined the role of phosphoinositide-3 kinase γ (PI3Kγ) signaling in the recruitment of inflammatory cells and bone marrow-derived fibroblasts into the kidney and development of renal injury and fibrosis in an experimental model of hypertension induced by angiotensin II.

RESULTS

Blood pressure was comparable between wild-type (WT) and PI3Kγ knockout (KO) mice at baseline. Angiotensin II treatment led to an increase in blood pressure that was similar between WT and PI3Kγ KO mice. Compared with WT mice, PI3Kγ KO mice were protected from angiotensin II-induced renal dysfunction and injury and developed less proteinuria. PI3Kγ deficiency suppressed bone marrow-derived fibroblast accumulation and myofibroblast formation in the kidney and inhibited total collagen deposition and extracellular matrix protein production in the kidney in response to angiotensin II. PI3Kγ deficiency inhibited the infiltration of F4/80+ macrophages and CD3+ T cells into the kidney and reduced gene expression levels of pro-inflammatory cytokines in the kidney following angiotensin II treatment. Finally, inhibition of PI3Kγ suppressed CXCL16-induced monocyte migration in vitro.

CONCLUSION

These results indicate that PI3Kγ mediates the influx of macrophages, T cells and bone marrow-derived fibroblasts into the kidney resulting in kidney injury and fibrosis.

The role of P2Y6R in cardiovascular diseases and recent development of P2Y6R antagonists

As a member of the P2Y receptor family with a typical 7-transmembrane structure, P2Y₆ purinergic receptor (P2Y₆R) belongs to the G-protein-coupled nucleotide receptor activating the phospholipase-C signaling pathway. P2Y₆R is widely involved in a range of human diseases, including atherosclerosis and other cardiovascular diseases, gradually attracting attention owing to its inappropriate or excessive activation. In addition, it was reported that P2Y₆R might regulate inflammatory responses by governing the maturation and secretion of proinflammatory cytokines. Hence, several P2Y₆R antagonists have been subjected to evaluation as new therapeutic strategies in recent years. This review was aimed at summarizing the role of P2Y₆R in the pathogenesis of cardiovascular diseases, with an insight into the recent progress on discovery of P2Y₆R antagonists.

ADP exerts P2Y12 -dependent and P2Y12 -independent effects on primary human T cell responses to stimulation

Purinergic signaling plays a complex role in inflammation. Nucleotides released by T lymphocytes, endothelial cells, and platelets during inflammation induce cellular responses by binding to receptors that regulate intracellular signaling pathways. Previous studies have found that purinergic signaling can have both proinflammatory and anti-inflammatory effects, but the roles of specific pathways in specific cell types are poorly understood. We investigated the role of the P2Y₁₂ signaling pathway in the activation of T lymphocytes in vitro. We isolated peripheral blood mononuclear cells (PBMCs) from healthy donors and pretreated them with ADP (a P2Y₁₂ agonist), AR-C69931MX (a P2Y₁₂ antagonist), or both. We then stimulated PBMC using phytohemagglutinin (PHA) or anti-CD3/CD28 antibodies. We found that ADP affects T cell responses in term of cell activity and receptor expression through both P2Y₁₂-dependent and P2Y₁₂-independent pathways and other responses (cytokine secretion) primarily through P2Y₁₂ -independent pathways. The ADP-mediated effect changed over time and was stimulus-specific.

Exacerbated intestinal inflammation in P2Y6 deficient mice is associated with Th17 activation

Extracellular nucleotides are released as constitutive danger signals by various cell types and activate nucleotide (P2) receptors such as P2Y₆ receptor. P2Y₆ activation on monocytes induces the secretion of the chemokine CXCL8 which may propagate intestinal inflammation. Also, P2Y₆ expression is increased in infiltrating T cells of Crohn's disease patients. As inflammatory bowel disease (IBD) is associated with immune cell recruitment, we hypothesised that P2Y₆ would participate to the establishment of inflammation in this disease. To address this, we used P2Y₆ deficient (P2ry6^--/-) mice in the dextran sodium sulfate (DSS) murine model of IBD. In disagreement with our hypothesis, P2Y₆ deficient mice were more susceptible to inflammation induced by DSS than WT mice. DSS treated-P2ry6^-/- mice showed increased histological damage and increased neutrophil and macrophage infiltration that correlated with increased mRNA levels of the chemokines KC and MCP-1. DSS treated-P2ry6^-/- mice exhibited also higher levels of Th17/Th1 lymphocytes in their colon which correlated with increased levels of IFN-γ and IL-17A in the sera as well as increased mRNA levels of IFN-γ, IL-17A, IL-6, IL-23 and IL-1β in P2ry6^-/- colons. This inflammation was also accompanied by a decreased cell proliferation and goblet cell number. Importantly, injection of anti-IL-17 intraperitoneally partially protected P2ry6^-/- mice from DSS-induced colitis. Taken together, in the absence of P2Y₆, an exacerbated intestinal inflammation to DSS was observed which correlated with increased recruitment of Th17/Th1 lymphocytes. These data suggest a protective role of P2Y₆ expressed on leukocytes in intestinal inflammation.

Inhibitors of DAG metabolism suppress CCR2 signalling in human monocytes

BACKGROUND AND PURPOSE

CCL2 is an inflammatory chemokine that stimulates the recruitment of monocytes into tissue via activation of the GPCR CCR2.

EXPERIMENTAL APPROACH

Freshly isolated human monocytes and THP-1 cells were used. Fura-2 loaded cells were used to measure intracellular Ca²⁺ responses. Transwell migration to measure chemotaxis. siRNA-mediated gene knock-down was used to support pharmacological approaches.

KEY RESULTS

CCL2 evoked intracellular Ca²⁺ signals and stimulated migration in THP-1 monocytic cells and human CD14⁺ monocytes in a CCR2-dependent fashion. Attenuation of DAG catabolism in monocytes by inhibiting DAG kinase (R59949) or DAG lipase (RHC80267) activity suppressed CCL2-evoked Ca²⁺ signalling and transwell migration in monocytes. These effects were not due to a reduction in the number of cell surface CCR2. The effect of inhibiting DAG kinase or DAG lipase could be mimicked by addition of the DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) but was not rescued by application of exogenous phosphatidylinositol 4,5-bisphosphate. Suppressive effects of R59949, RHC80267, and OAG were partially or fully reversed by Gö6983 (pan PKC isoenzyme inhibitor) but not by Gö6976 (PKCα and PKCβ inhibitor). RNAi-mediated knock-down of DAG kinase α isoenzyme modulated CCL2-evoked Ca²⁺ responses in THP-1 cells.

CONCLUSIONS AND IMPLICATIONS

Taken together, these data suggest that DAG production resulting from CCR2 activation is metabolised by both DAG kinase and DAG lipase pathways in monocytes and that pharmacological inhibition of DAG catabolism or application suppresses signalling on the CCL2-CCR2 axis via a mechanism dependent upon a PKC isoenzyme that is sensitive to Gö6983 but not Gö6976.

P2Y12 receptor modulation of ADP-evoked intracellular Ca2+ signalling in THP-1 human monocytic cells

BACKGROUND AND PURPOSE

The Gi -coupled, ADP-activated P2Y12 receptor is well characterized as playing a key role in platelet activation via crosstalk with the P2Y1 receptor in ADP-evoked intracellular Ca2+ responses. However, there is limited knowledge on the role of P2Y12 receptors in ADP-evoked Ca2+ responses in other blood cells. Here, we investigated the role of P2Y12 receptor activation in the modulation of ADP-evoked Ca2+ responses in human THP-1 monocytic cells.

EXPERIMENTAL APPROACH

A combination of intracellular Ca2+ measurements, RT-PCR, immunocytochemistry, leukocyte isolation and siRNA-mediated gene knockdown were used to identify the role of P2Y12 receptor activation.

KEY RESULTS

ADP-evoked intracellular Ca2+ responses (EC50 2.7 μM) in THP-1 cells were abolished by inhibition of PLC (U73122) or sarco/endoplasmic reticulum Ca2+ -ATPase (thapsigargin). Loss of ADP-evoked Ca2+ responses following treatment with MRS2578 (IC50 200 nM) revealed a major role for P2Y6 receptors in mediating ADP-evoked Ca2+ responses. ADP-evoked responses were attenuated either with pertussis toxin treatment, or P2Y12 receptor inhibition with two chemically distinct antagonists (ticagrelor, IC50 5.3 μM; PSB-0739, IC50 5.6 μM). ADP-evoked responses were suppressed following siRNA-mediated P2Y12 gene knockdown. The inhibitory effects of P2Y12 antagonists were fully reversed following adenylate cyclase inhibition (SQ22536). P2Y12 receptor expression was confirmed in freshly isolated human CD14+ monocytes.

CONCLUSIONS AND IMPLICATIONS

Taken together, these data suggest that P2Y12 receptor activation positively regulates P2Y6 receptor-mediated intracellular Ca2+ signalling through suppression of adenylate cyclase activity in human monocytic cells.

ATP Evokes Ca2+ Responses and CXCL5 Secretion via P2X4 Receptor Activation in Human Monocyte-Derived Macrophages

Leukocytes sense extracellular ATP, a danger-associated molecular pattern, released during cellular stress and death, via activation of cell surface P2X and P2Y receptors. Here, we investigate P2 receptor expression in primary human monocyte-derived macrophages and receptors that mediate ATP-evoked intracellular [Ca²⁺]_i signals and cytokine production in response to ATP concentrations that exclude P2X₇ receptor activation. Expression of P2X₁, P2X₄, P2X₅, P2X₇, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, and P2Y₁₃ was confirmed by quantitative RT-PCR and immunocytochemistry. ATP elicited intracellular Ca²⁺ responses in a concentration-dependent fashion (EC₅₀ = 11.4 ± 2.9 μM, n = 3). P2Y₁₁ and P2Y₁₃ activations mediated the amplitude of [Ca²⁺]_i response, whereas P2X₄ activation, but not P2X₁ or P2X₇, determined the duration of Ca²⁺ response during a sustained phase. ATP mediated gene induction of CXCL5, a proinflammatory chemokine. P2X₄ antagonism (PSB-12062 or BX430) inhibited ATP-mediated induction of CXCL5 gene expression and secretion of CXCL5 by primary macrophage. Inhibition of CXCL5 secretion by P2X₄ antagonists was lost in the absence of extracellular Ca²⁺ Reciprocally, positive allosteric modulation of P2X₄ (ivermectin) augmented ATP-mediated CXCL5 secretion. P2X₇, P2Y₁₁, or P2Y₁₃ receptor did not contribute to CXCL5 secretion. Together, the data reveals a role for P2X₄ in determining the duration of ATP-evoked Ca²⁺ responses and CXCL5 secretion in human primary macrophage.

Deconstructing metabolic inflammation using cellular systems

Over the past years, we have embarked in a systematic analysis of the effect of obesity or fatty acids on circulating monocytes, microvascular endothelial cells, macrophages, and skeletal muscle cells. With the use of cell culture strategies, we have deconstructed complex physiological systems and then reconstructed "partial equations" to better understand cell-to-cell communication. Through these approaches, we identified that in high saturated fat environments, cell-autonomous proinflammatory pathways are activated in monocytes and endothelial cells, promoting monocyte adhesion and transmigration. We think of this as a paradigm of the conditions promoting immune cell infiltration into tissues during obesity. In concert, it is possible that muscle and adipose tissue secrete immune cell chemoattractants, and indeed, our tissue culture reconstructions reveal that myotubes treated with the saturated fatty acid palmitate, but not the unsaturated fatty acid palmitoleate, release nucleotides that attract monocytes and other compounds that promote proinflammatory classically activated "(M1)-like" polarization in macrophages. In addition, palmitate directly triggers an M1-like macrophage phenotype, and secretions from these activated macrophages confer insulin resistance to target muscle cells. Together, these studies suggest that in pathophysiological conditions of excess fat, the muscle, endothelial and immune cells engage in a synergistic crosstalk that exacerbates tissue inflammation, leukocyte infiltration, polarization, and consequent insulin resistance.

P2Y6 Receptor Antagonist MRS2578 Inhibits Neutrophil Activation and Aggregated Neutrophil Extracellular Trap Formation Induced by Gout-Associated Monosodium Urate Crystals

Human neutrophils (polymorphonuclear leukocytes [PMNs]) generate inflammatory responses within the joints of gout patients upon encountering monosodium urate (MSU) crystals. Neutrophil extracellular traps (NETs) are found abundantly in the synovial fluid of gout patients. The detailed mechanism of MSU crystal-induced NET formation remains unknown. Our goal was to shed light on possible roles of purinergic signaling and neutrophil migration in mediating NET formation induced by MSU crystals. Interaction of human neutrophils with MSU crystals was evaluated by high-throughput live imaging using confocal microscopy. We quantitated NET levels in gout synovial fluid supernatants and detected enzymatically active neutrophil primary granule enzymes, myeloperoxidase, and human neutrophil elastase. Suramin and PPADS, general P2Y receptor blockers, and MRS2578, an inhibitor of the purinergic P2Y6 receptor, blocked NET formation triggered by MSU crystals. AR-C25118925XX (P2Y2 antagonist) did not inhibit MSU crystal-stimulated NET release. Live imaging of PMNs showed that MRS2578 represses neutrophil migration and blocked characteristic formation of MSU crystal-NET aggregates called aggregated NETs. Interestingly, the store-operated calcium entry channel inhibitor (SK&F96365) also reduced MSU crystal-induced NET release. Our results indicate that the P2Y6/store-operated calcium entry/IL-8 axis is involved in MSU crystal-induced aggregated NET formation, but MRS2578 could have additional effects affecting PMN migration. The work presented in the present study could lead to a better understanding of gouty joint inflammation and help improve the treatment and care of gout patients.

Purinergic Signaling During Immune Cell Trafficking

Migration and positioning of immune cells is fundamental for their differentiation and recruitment at sites of infection. Besides the fundamental role played by chemokines and their receptors, recent studies demonstrate that a complex network of purinergic signaling events plays a key role in these trafficking events. This process includes the release of nucleotides (such as ATP and ADP) and subsequent autocrine and paracrine signaling events through nucleotide receptors. At the same time, surface-expressed ectoapyrases and nucleotidases convert extracellular nucleotides to adenosine, and adenosine signaling events play additional functional roles in leucocyte trafficking. In this review we revisit classical paradigms of inflammatory cell trafficking in the context of recent studies implicating purinergic signaling events in this process.

Extracellular granzyme K mediates endothelial activation through the cleavage of protease-activated receptor-1

Granzymes are a family of serine proteases that were once thought to function exclusively as mediators of cytotoxic lymphocyte-induced target cell death. However, non-apoptotic roles for granzymes, including granzyme K (GzK), have been proposed. As recent studies have observed elevated levels of GzK in the plasma of patients diagnosed with clinical sepsis, we hypothesized that extracellular GzK induces a proinflammatory response in endothelial cells. In the present study, extracellular GzK proteolytically activated protease-activated receptor-1 leading to increased interleukin 6 and monocyte chemotactic protein 1 production in endothelial cells. Enhanced expression of intercellular adhesion molecule 1 along with an increased capacity for adherence of THP-1 cells was also observed. Characterization of downstream pathways implicated the mitogen-activated protein kinase p38 pathway for intercellular adhesion molecule 1 expression, and both the p38 and the extracellular signal-regulated protein kinases 1 and 2 pathways in cytokine production. GzK also increased tumour necrosis factor α-induced inflammatory adhesion molecule expression. Furthermore, the physiological inhibitor of GzK, inter-α-inhibitor protein, significantly inhibited GzK activity in vitro. In summary, extracellular GzK promotes a proinflammatory response in endothelial cells.